In a general method of assembling components on printed circuit boards, the component leads are mounted through holes punched in the board for that purpose and the leads are then clinched, or bent, to hold the components in place during subsequent fluxing, lead cutting and soldering operations. The leads are soldered to the boards and the excess leads cut off. In many cases, the cut lead ends must be resoldered to cover them. These operations require hand labor and many steps, which is of course expensive. Also, solder is lost on the discharged lead ends. Further, when a double soldering step is required, the danger of board warping, due to the high temperatures involved in soldering, is increased.
With the increased complexity of electronic devices, more components are mounted on the boards, which creates more problems. The clinched leads on the boards interfere with each other, necessitating a stepwise component mounting whereby part of the components are assembled, soldered and their leads cut off before mounting additional components.
Further, it is more difficult to remove faulty components which must be replaced or repaired when the leads have been clinched, without damaging the board or other adjacent components. Still further, access to both sides of the board is required.
Thus, other ways of assembling printed circuit boards components without clinching leads have been sought. U.S. Pat. Nos. 2,973,499 and 3,230,612 describe adaptors or griplets which are fitted into the board holes and which receive the lead ends and pin or grip them to hold them in place. However, these methods require additional steps and parts which also add to the expense of manufacture.
U.S. Pat. No. 3,540,718 describes a component clamp and jig assembly whereby a sliced compressable pad, as of plastic foam, is pressed against the components to keep them in place during the soldering and lead cutting operation. This method is not entirely satisfactory either because it is difficult to maintain a uniform pressure against all components, which may differ greatly is size and weight. Also, the pad tends to become impregnated with flux and solder after several operations and requires extensive cleaning to retain its resiliency.
Other means of holding the components down by means of pressure have also been tried. When all the components are of nearly uniform size and weight, a bag loaded with small particles, e.g., beans or plastic shot, placed over the components will help keep them in place. However, this method is also unsatisfactory when both large and small components must be assembled in close proximity to each other, because the smaller components will not be held down properly and they may become loosened or even dislodged during soldering.
According to another method that has been tried, a molten wax coating is applied to the bottom of the board after the component leads have been inserted into the holes. After the wax cools and hardens, it holds the components in place while the leads are cut and soldered. This method has several advantages; it is particularly adapted to automatic soldering operations and the leads can be cut prior to soldering. The solder then covers all the lead ends in one operation. In addition, the wax acts as a flux for the soldering operation. The problems with the existing wax material are that the wax smokes during soldering, which is annoying to the operator, and the wax must be removed, either with organic solvents or with hot water, which requires a temperature over 150.degree. F. (65.56.degree. C.) which is expensive. In some instances also the soldering efficiency of the wax is inadequate for automatic soldering operations, necessitating the application of a second flux over the wax coating.
Applications Ser. Nos. 771,680, filed Feb. 24, 1977, and 771,679, filed Feb. 24, 1977, assigned to the same assignee, solve the above problems through the use of an extrudable, non-flowable, completely water soluble composition that can be readily applied over the components assembled on the printed circuit board to hold the components in place on the wiring board without lead clinching during the soldering operation. The compositions are suitable for the generally more acidic water soluble fluxes and water rinses but, due to insolubility in the non-aqueous diluent-solvents, are unsuitable if milder alcohol-soluble rosin fluxes and solvent rinses are necessary to prevent component corrosion. The terms printed circuit board and wiring board are interchangeable.